(1) Field of the Invention
The present invention generally concerns techniques allowing the detection of any defects in a product moving in a rolling mill installation.
More precisely, the invention according to one first aspect concerns a defect detection method comprising at least one illuminating operation consisting of illuminating the moving product with a light source, an acquisition operation consisting of forming by scanning a first image of the moving product, as observed in a first spectral band of light, a pre-processing operation consisting at least of producing, from the first image of an observation area of the product, a first digitised luminosity distribution of this region, a detection and extraction operation of suspect zones consisting of using the first digitised luminosity distribution of the observation area of the product to detect the presence and location of any defects in this area, and a classification operation consisting at least of classifying suspect zones into one or more categories of defects or non-defects by comparison of the morphological and/or photometric characteristics of these suspect zones with morphological and/or photometric characteristics of known defects and of known non-defects, held in a pre-established database.
The digitised luminosity distribution is formed of pixels encoded over several bits and forms a normalized grey scale image corresponding to the distribution of luminosity of the image resulting from the acquisition, after correction of known distortions inherent in lighting and image taking.
(2) Prior Art
A method of this type is known to persons skilled in the art for cold rolled steel products, this method being particularly described in an article by Mr. Dominique Blanchard entitled “Automatic surface inspection system: experience and perspectives within the Usinor Group” published in June 2002 in “Revue de la Métallurgie” (Journal of Metallurgy).
This known method is of major interest for real-time inspection of cold-rolled steel products called “flats” i.e. products having a very high width/thickness ratio, typically greater than 100 or over.
Having regard to the very narrow thickness of these products, any defects which may affect them are likely to cause practically always deteriorations of their outer surface.
The situation is very different for products known as “long products” in steel-making, which are characterized by a section of geometric shape whose two main dimensions have a size of much the same magnitude, and which can be affected not only by surface defects but also by inner defects that are not revealed on the surface.
Long products are generally obtained from bulk billets that are continuously cast and then hot rolled in a succession of rolling mill stands arranged horizontally and vertically, these directions, for each mill stand, defining the orientation of the rotation axis of its working rolls. Rolling is conducted at high temperature, generally at around 1000° C. for steel, the invention not being limited however to steel and possibly being applied to other metals, in particular non-ferrous metals such as copper, or even to other materials.
The rolling of steel allows the size of the product to be reduced in two perpendicular directions while lengthening the product along the axis of the third direction. With this method it is possible to obtain special mechanical properties with a certain axial symmetry. Depending on the geometric shape of the section to be obtained, a greater or less number of mill stands are provided, and special section shapes can be achieved by using fluted rolls. The succession of section shapes of the product during lengthening, called roll pass design, plays a particularly important role to obtain the mechanical characteristics of the product in all the directions of this section. It is therefore possible to obtain products in bar form which may have a round or oval section, or even square, rectangular, hexagonal or octagonal. However, more complex, non-symmetric section shapes can also be achieved such as angle bars, I- or H-beams or rails.
Nonetheless said method had its disadvantages for the structure of the product, metal products in particular and especially in steel.
Defects of different origins may effectively come to be associated within the structure of the product, in particular surface defects due to mill scale which continues to adhere to the product surface and is caused by oxidation of the surface caused by water cooling of the mill stands. Since rolling is performed in two perpendicular directions in the horizontal stands and in the vertical stands, the scale may also penetrate inside the product with much more ease than in flat sheets whose thickness is always reduced in the same direction and is substantially reduced with respect to their width. Additionally, the use of fluted rolls for some mill stands may cause metal folds, which can cause some quantities of metal of different temperature to enter inside the product section thereby giving rise to heterogeneities. Finally, more upstream difficulties, such as poor lubrication when casting the billet or heat shock at the time of cooling, may generate structural defects of the metal which may come to be located inside the section of the product as well as on its surface after rolling.
It is therefore important, in particular for long products, to be able to detect not only surface defects but also defects present under the surface.
A technique to detect inner defects of a product is already known, based on analysis of the propagation of ultrasound waves inside this product. This technique, known as the Lamb wave technique, gives satisfactory results for the detection of inclusions, but requires the transmitter-sensor assembly to be in contact with the product to be inspected via an oil-water liquid layer to ensure proper transmission of the ultrasound waves. This constraint evidently means that the technique under consideration cannot be applied when the product to be inspected consists of a mass of steel at high temperature.